There is conventionally known WCDMA as one of typical communication methods used in mobile communication systems. According to the WCDMA, a plurality of channels uses the same frequency, so that the channels are often faced with interferences from the other channels.
Such an interference of the other channels according to the WCDMA is generally greater if a transmission power from a base station is higher. Since it is known that this interference is a main factor for deciding a line capacity of the base station, it is desirable to reduce the transmission power from the base station as lower as possible in view of the capacity of the line to be used.
Meanwhile, a communication quality of a communication between the mobile device or mobile terminal and the base station depends on SIR (Signal to Interference Ratio). Namely, if the transmission power from the base station is higher, the SIR is higher and the communication quality is improved. If the transmission power from the base station is lower, the SIR is lower and the communication quality is degraded.
It is, therefore, necessary to control a magnitude of a target SIR of the mobile device or mobile terminal so as to cause the mobile device or mobile terminal to satisfy an appropriate communication quality according to a status of a communication environment and to reduce the transmission power from the base station.
Outer loop transmission power control normally used in the WCDMA is intended to optimize the line capacity and the communication quality by controlling the transmission power from the base station so that the communication quality of the mobile device or mobile terminal is equal to a desired communication quality designated by network side.
Specifically, the target SIR of the mobile terminal is controlled so that a BLER (Block Error Rate) indicating the communication quality between the base station and the mobile terminal is equal to a target BLER designated by a network where the base station is present, and the mobile terminal indirectly controls the transmission power from the base station.
As this target SIR control method, a cellular system transmission power control method including causing a receiver side to detect a frame error, calculating the number n of frame errors indicating the number of frames until m frame errors are detected whenever the m frame errors are detected, and increasing or decreasing a target SIR according to the number n of frame errors is disclosed (see Patent Document 1).
Patent Document 1: Japanese Patent Application Laid-Open No.